Remember that tree levels are generated by context free languages, not
regular languages, and so they can only be recognized by push-down
automatons, not finite state machines.
What happened was that I failed to account for transparent rules.
Without transparent rules the children of a tree node do not have any
recursion in them (by definition!) and so therefore *are* a regular
language. But transparent rules change that: there *can be* recursion
hidden on the same tree level, and it should have been clear from a
moment's reflection that the recursion there meant that tree levels
were once again a context free language.
Fortunately we have a recognizer for context free languages lying
around, so we can just use that I guess.
Still very garbage but I think the "hard" part of building a Wadler
document from a parse tree might be there. It's a backtracking matcher
which might turn out to be too slow for alternatives but maybe will be
fine?
Still needs lots of tests.
